Results 1 - 10 of 402
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[en] Resonant absorption is considered to be a crucial mechanism for the damping of the coronal loop oscillations and plasma heating. We study resonant absorption of the coronal loop kink oscillations excited by such external drivers as flares, assuming that there is an intermediate shear flow region surrounding the loop. We find that for long coronal loops resonant absorption can be highly enhanced or reduced depending sensitively on the magnitude and direction of the flow and the spatial extent of the flow region when the transitional layer is thin. For short coronal loops, high flow speed and a thick transitional layer are needed to have a substantial resonant absorption. We provide a potential picture to explain the results where the external Alfvén speed and phase speed of the wave are important parameters. These results imply that the transport of the external wave energy into the loop is significantly changed by the shear flow region, which may cause the selective excitation of the coronal loop oscillations.
[en] Calculations of accurate atomic data required for the determination of stellar-envelope opacities are made in a collaborative effort which is referred to as the Opacity Project. This report discusses the theoretical background for these calculations and some results. 28 refs.; 11 figs
[en] We present the results of spectral analysis of a sample of RS CVn stars observed with the EXOSAT Observatory. Data from both the Medium Energy and Low Energy experiments are analysed assuming simple spectral models (one-temperature, two-temperature, continuous emission measure distribution models). We investigate the dependence of the fitted coronal parameters on the signal-to-noise ratio and the instrument response function, and compare the results with those previously obtained by the Einstein IPC and SSS experiments. We find evidence for multi-temperature coronal structures in RS CVn stars as well as indications of intrinsic differences in the temperature stratification of different stars. The simple power law EM ∼ Tα assumed by us for the continuous emission measure distribution appears to be inconsistent with several of our observations. For the others, we derive power law indices α that are higher than those found for solar coronal loops and interpret this as evidence that the coronae of RS CVn binaries involve structures different from those typically observed on the Sun
[en] Numerical calculations are presented of the dynamical response of a corona when the mechanical flux FOmech, that heats the corona by dissipation, is instantaneously increased or decreased. An implicit conservative upwind method is used. The corona shows a temporary unstable behaviour when the transition region comes out of energy balance. Thermal conduction quickly stabilizes the instability and the corona finds a new stationary state as soon as wave fronts generated by the instability and the changed pressure scale height have propagated out of the numerical grid. The response of the corona is explained in terms of its physical time constants. The stability of minimum flux coronae is rediscussed in the light of these calculations, and some of the criticisms of that theory are shown to be invalid
[en] We consider the structure of a particular class of 2D x-invariant magnetostatic equilibria in which the magnetic field is potential everywhere in some domain of space but on some singular surfaces - current-sheets - carrying a non-zero net x-current. We establish some new general properties of configurations of this type, and discuss a variational principle allowing to compute them
[en] It is well known that some coronal jets exhibit helical structures and untwisting. We attempt to inspect the origin of twist in a blowout jet. By means of multi-wavelength and multi-angle observations from Solar Dynamics Observatory (SDO) and Solar Terrestrial Relations Observatory-Ahead (STEREO-A), we firstly report a polar untwisting jet that is a blowout jet which leads to a jet-like coronal mass ejection. From the viewpoint of SDO, the jet shows clear untwisting behavior and two jet-spires. However, from the viewpoint of STEREO-A the jet actually comes from the whiplike prominence eruption and is followed by a white-light jet. Our observations indicate that twist in blowout jets may result from the erupting mini-prominences/mini-filaments in the jet base. (letters)
[en] In solar filament formation mechanisms, magnetic reconnection between two sets of sheared arcades forms helical structures of the filament with numerous magnetic dips, and cooling and condensation of plasma trapped inside the helical structures supply mass to the filament. Although each of these processes, namely, magnetic reconnection and coronal condensation have been separately reported, observations that show the whole process of filament formation are rare. In this Letter, we present the formation of a sigmoid via reconnection between two sets of coronal loops, and the subsequent formation of a filament through cooling and condensation of plasma inside the newly formed sigmoid. On 2014 August 27, a set of loops in the active region 12151 reconnected with another set of loops that are located to the east. A longer twisted sigmoidal structure and a set of shorter lower-lying loops then formed. The observations coincide well with the tether-cutting model. The newly formed sigmoid remains stable and does not erupt as a coronal mass ejection. From the eastern endpoint, signatures of injection of material into the sigmoid (as brightenings) are detected, which closely outline the features of increasing emission measure at these locations. This may indicate the chromospheric evaporation caused by reconnection, supplying heated plasma into the sigmoid. In the sigmoid, thermal instability occurs, and rapid cooling and condensation of plasma take place, forming a filament. The condensations then flow bidirectionally to the filament endpoints. Our results provide a clear observational evidence of the filament formation via magnetic reconnection and coronal condensation.
[en] A flare on the dMe star AD Leonis was simultaneously observed with the radio telescopes in Effelsberg, Jodrell Bank, and Arecibo using spectrometers at 1665, 166 and 1415 MHz with bandwidths of 25, 100 and 40 MHz respectively. The time coincidence confirms the stellar origin of the radiation. The flare emission was resolved into a multitude of broadband pulsations. The e-folding rise and decay times were of the order of the time resolution of the Effelsberg data (125 ms), or less. The circular polarization was ∼ 100%. Similar bursts, but 4 orders of magnitude less powerful, have been observed from the sun at lower frequencies
[en] The ratio of the emission line fluxes for the C II and CIV lines in the lower transition regions (3x104 < T < 105K) between stellar chromospheres and transition layers depends mainly on the temperature gradient in the line emitting regions which can therefore be determined from this line ratio. The temperature gradient is determined by the equilibrium between energy input and energy loss. From the observed temperature gradient we can therefore determine the temperature dependence of the energy input, which is expected to be different for different energy input mechanisms. We study the flux ratios of the C II to CIV emission line fluxes in order to obtain information about the energy input mechanism